I am planning to make a double blanket with my handspun for weft. I am confused on how to get my warp threads aligned if I am doing a 8 shaft weave as I usually use a tabby but when using 8 shafts a tabby will "glue the double weave together and when I take it off of the loom it will be able to open.
You need to determine your tabby for each layer. Your double blanket will be composed of two layers, lets say for the sake of example, one layer on 8 shafts and the other layer on the next 8 shafts. For each layer you determine which shafts will create tabby. When you weave tabby on the top layer you simply lift the shafts required, but when you weave it on the bottom layer you have to lift the shafts required PLUS all the shafts for the top layer to keep them out of the way. You cannot weave "tabby" with every other thread in the warp or you will do what you say, glue the layers together. If you don't have 16 shafts you cannot do an 8 shaft pattern in double weave. If you have 8 shafts you can do pattern with four shafts for each layer, and each layer will have its own tabby. It would help to know what pattern you are trying to weave and how many shafts you have available.